In a principal aspect, the present invention relates to high-strength, low-hysteresis shape-memory alloys (SMAs), and in particular TiNi-based SMAs, employing coherent, low-misfit nanoscale-sized precipitates. Such alloys are contemplated to have a myriad of practical applications including, but not limited to, use in medical stents and actuators.
The shape-memory effect is a consequence of a crystallographic reversible, thermoelastic martensitic transformation. SMAs rely on property changes induced during the transformation from a high temperature phase (parent phase) to a low temperature phase (product phase or martensite); the product phase is relatively compliant in comparison with the parent phase.
Shape-memory actuation occurs when an SMA is deformed in its martensite state, below its Ms temperature; the deformed shape is maintained upon unloading. Once reheated beyond the austenite finish temperature (Af), an SMA will work against a resisting force to regain its original shape.
Superelasticity occurs when an SMA is deformed above austenite start temperature (As), but below Msσ (the highest temperature possible to have martensite). In this range, martensite can be made stable with the application of stress, but becomes unstable again when the stress is removed. Because of superelasticity, SMAs can deform elastically up to large strains and recover perfectly without being damaged by unloading, similar to rubber.
Under constrained conditions, the output stress of an SMA during reversion of martensitic transformation is typically limited by the flow strength of the parent phase. For engineering applications, it is also highly desirable, if not essential, that the shape-memory behavior is repeatable and predictable after many cycles through the transformation. Therefore, to improve both the output force and the cyclic lifetime of SMAs, the strength of the alloy (i.e. flow strength of parent phase) must be improved. By raising the critical shear stress for slip, the irreversible slip deformation during the martensite reorientation and stress-induced martensite transformation can be suppressed, which, in turn, improves the shape-memory effect and transformation superelasticity characteristics.
Currently, the three most commonly used SMAs in engineering applications are TiNi and the copper-based alloys, CuZnAl and CuAlNi. Several iron-based SMAs, such as FeMnSi, FePt, and FePd are also the subject matter of research for industrial applications. However, TiNi-based alloys are currently the most widely used SMAs due to good corrosion resistance and biocompatibility.
Various types of precipitate strengthening may be considered in the TiNi-based system. On the Ti-rich side of the binary Ti-Ni system, a Ti2Ni dispersion can be obtained while on the Ni-rich side Ni3Ti/Ni4Ti3 precipitates can be considered for strengthening dispersions. Kajiwara et al. [Philos. Mag. Lett., 1996, vol. 74, pp. 137-144, J. Phys. IV, 2001, vol. 11, pp. 395-405, and Metall. Mater. Trans. A, 1997, vol. 28, pp. 1985-1991 (incorporated herewith)] found that subnanometric thin plate metastable bct precipitates formed when sputter-deposited Ti-rich TiNi shape-memory films are annealed in the temperature range of 377 to 827° C. Due to the relatively low heat treatment temperature, diffusion of Ti atoms is not rapid enough to form stable Ti2Ni precipitates; instead, Guinier-Preston zone-type precipitates which contain excess Ti atoms are produced. With these fine precipitates in the parent phase, they could achieve recovery strength 670 MPa. However, these precipitates have been observed only after annealing of sputter deposited and amorphous TiNi thin films.
While precipitation strengthening has been mainly considered in TiNi-base thin films or bulk single crystals, deformation processing has been considered in bulk polycrystalline TiNi alloys for strengthening. Lin and Wu [Acta Metall. Mater., 1994, vol. 42, pp. 1623-1630 (incorporated herewith)] studied cold-rolled equiatomic TiNi alloys. With a cold rolling at room temperature to the extent of 31% reduction in thickness, they improved the yield stress from 380 MPa of a solution treated specimen to 1000 MPa. However this is an economically inefficient approach, as the alloys have to be heat-treated following each cold rolling step.
Koizumi et al. [Mater. Sci. Engng A.: 1997, vol. 223, pp. 36-41 (incorporated herewith)] examined the high-temperature strength of TiNi alloys in the context of developing new alloys to replace Ni-base superalloys. They demonstrated that a dispersion of Heusler phase (Ni2TiAl-type with L21 structure) increases the compressive yield strength of 50.7Ni-40.9Ti-8.4Al (in at %) by an order of magnitude up to 2300 MPa. This strengthening method is potentially applicable to both thin film and bulk alloy processing. While they have achieved impressive compressive yield strength in the TiNi-based alloys with Heusler precipitates, they did not consider any of the shape-memory characteristics of the alloys. Their alloys were solely developed as high-temperature materials, neglecting the thermoelastic transformations and the superelasticity.
In CuZnAl-based SMAs, a ductile second phase α can be distributed within the β matrix. Compared with that of the single-phase alloy, the fatigue life of dual-phase alloys with homogeneously distributed globular α phase is increased in both the martensitic and superelastic state [Metall. Trans. A, 1992, vol. 23, pp. 2939-2941 (incorporated herewith)]. Semi-coherent γ precipitates in the α matrix have been studied by Lovey and Cesari [Mater. Sci. Engng A.: 1990, vol. 129, pp. 127-133 (incorporated herewith)]. In CuAlNi-based SMAs, the precipitation of coherent γ2 intermetallic compound (Cu9Al4) can be considered [J. Phys. IV, 1995, vol. 5 (C2), pp. 193-197 (incorporated herewith)]. In FeMnSi-based SMAs, the addition of small amounts of Nb and C is known to produce very small NbC carbide precipitates in austenite, which improves the shape memory effect [Scripta Mater., 2001, vol. 44, pp. 2809-2814 (incorporated herewith)]. Various phases useful for strengthening the parent phase of the matrix are summarized in TABLE 1.
TABLE 1SMA Parent PhaseStrengthening PhasesB2-TiNiMetastable Ti2Ni, Ni4Ti3,Stable Ni3Ti, L12 (Ni2TiAl)β-CuZnAlα, γβ-CuAlNiγ2 (Cu9Al4)FeMnSiNbC
Nonetheless, microstructural design and the implementation in processing for improving the strength of SMAs while controlling the transformation temperatures have remained a scientific and engineering challenge. High-strength, low-hysteresis TiNi-based SMAs as well as other SMAs which achieve yield strength greater than 1200 MPa while maintaining desired transformation temperatures are much needed.